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Corrosion of silicon nitride in sulphuric acid aqueous solution
Corrosion Science 43 (2001) 1851±1863
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Corrosion of silicon nitride in sulphuric acid aqueous solution F. Monteverde, C. Mingazzini, M. Giorgi, A. Bellosi * CNR-IRTEC, Research Institute for Ceramic Technology, Via Granarolo 64, 48018 Faenza, Italy Received 21 January 2000; accepted 13 May 2000
Abstract The corrosion of a hot pressed silicon nitride (produced with the addition of 3 wt.% Y2 O3 3 wt.% La2 O3 as sintering aids) in 1.8 M sulphuric acid aqueous solution was studied in the temperature range of 25±70°C for times up to 400 h. No reaction products were detected on the exposed surfaces. The corrosion follows a progressive chemical dissolution of the La±Y±Si±O±N grain boundary phases: their basic character, due to the presence of Y and La cations, favours the attack by acid aqueous environments. The reaction front enters irregularly into the bulk and depletes an external layer and inner zones of the samples of grain boundary phases. At 25°C up to 100 h the corrosion kinetics is linear: the chemical reaction is rate limiting. At 40°C and 70°C the kinetics ®ts a three-dimensional diusion law: the mass transport of the reactants to the reaction sites and/or the removal of products from the reacting interface(s) are rate limiting. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Structural ceramic; Silicon nitride; Acid corrosion; Weight change; Corrosion kinetics
1. Introduction In last decades, the applications of silicon-nitride-based materials in many engineering applications, among which corrosion- and wear-resistant components, have gained increasing attention, due to their unique combination of properties: chemical stability, thermal shock resistance, hardness, fracture strength, toughness. A lot of technical and economic resources has been actively applied on high temperature
*
Corresponding author. Tel.: +39-546-699-759; fax: +39-546-46381. E-mail address: [email protected] (A. Bellosi).
0010-938X/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 0 - 9 3 8 X ( 0 0 ) 0 0 1 9 3 - 1
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corrosion and oxidation studies of silicon nitride [1±3], whereas fewer papers have concerned the corrosion behaviour at room or moderate temperatures in aqueous corrosive environments [4±21]. Silicon-nitride-based ceramics are multiphase systems with each phase having individual corrosion characteristics and possible reactions at the grain boundaries. It was reported that corrosion of silicon-nitride-based materials varies, depending on the acid or basic conditions and on the residual grain boundary phases of the as-sintered materials (i.e. structure-composition, amount, distribution, degree of crystallinity). As the microstructural features of the grain boundary phases are directly correlated to type, amount of sintering aids and to the adopted manufacturing procedures (powder processing, cold and hot consolidation techniques), various silicon-nitride-based materials showed dissimilar corrosion behaviour and, consequently, dierent eects on structural and functional properties [5,6,9,11,13,14,16±21]. The purpose of the present study is to investigate the corrosion behaviour in 1.8 M sulphuric acid solution of an hot pressed silicon nitride material containing yttria and lanthania as sintering aids. The kinetics features were evaluated and the mechanisms governing the corrosion were de®ned. The in¯uence of the microstructure of the as-sintered material on the corrosion resistance was considered. 2. Experimental procedures A fully dense hot pressed silicon-nitride-based material (containing 3 wt.% Y2 O3 3 wt.% La2 O3 as sintering aids) was employed for corrosion tests. Details on processing and main properties of this material are reported elsewhere [22]. For corrosion tests rectangular plates 10:0 10:0 2:0 mm3 were cut and the wider surfaces were polished up to 6 lm. After polishing, the plates were carefully cleaned in ultrasonicated acetone bath, dried, weighed with an analytical balance (accuracy 0.01 mg) and sealed in a 100 ml polyethylene tube containing 50 ml of 1.8 M sulphuric acid (pH < 0). The tubes were then immersed in a thermostatic bath and gently shaken twice a day. The static corrosion tests were performed at 25°C, 40°C and 70°C for holding times up to 400 h. After the planned exposure time, the specimens were removed from the tubes, rinsed in boiling deionized water, dried in warm heated oven and ®nally weighed. For each experimental condition, at least two tests were performed. The determination of the amount of Si, Y and La ions released in the corrosive solutions was carried out with a Varian-Liberty 200 sequential inductively coupled plasma±atomic emission spectroscopy (ICP±AES). Not withstanding the high acid concentration, adopting a V-groove nebulizer and speci®c conditions (low nebulization pressure and high power applied to the plasma) short-term and long-term stability of signals were both satisfactory. In order to ensure accuracy, matrixmatched standard solutions were prepared. Without any dilution, the concentrations resulted much higher than the (lower) detection limit on the selected analytical lines (La: 408.67 nm; Y: 371.03 nm; Si: 251.61 nm). However the use of a demountable torch with an alumina injector was necessary in order to minimise the signal of
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silicon arising from the quartz parts of the torch itself. The standard solutions were tested over a period of two months to exclude adsorption/precipitation phenomena upon the walls of the tubes. Microstructural investigations of the samples after corrosion were performed by X-ray diraction (XRD) and scanning electron microscopy (SEM) on the surfaces and relative cross sections in order to evaluate the eects of the attack on surface and inside the bulk.
3. Results and discussion The typical microstructure of the hot pressed material is shown in Fig. 1 with elongated and interlocked b-silicon nitride grains (grey areas) and grain boundary phases (bright network). The grain boundary phases are mainly glassy; after annealing treatments La±Y±Si±O±N phases of various stoichiometries were detected by XRD patterns [22]. These results allowed to get informations on the qualitative composition of the residual grain boundary phases of the tested specimens. While neither noticeable dimensional change nor delamination of silicon nitride grains were observed after exposure, all corroded samples suered extensive whitening from the original dark-grey colour all over the surfaces wet by the acid sulphuric aqueous solution. The weight loss data of the corroded samples at 25°C, 40°C and 70°C for holding times up to 400 h are shown in Fig. 2: each tested sample had signi®cant weight loss. XRD patterns from the corroded surfaces did not reveal any additional crystalline phases in comparison to the as-sintered material. On the exposed surfaces only small and mildly leached areas, where grain boundary phases were selectively dissolved, appeared (Fig. 3). The surface aspect was similar in all the samples treated under dierent experimental conditions.
Fig. 1. SEM micrograph by secondary electrons (SE) from a polished and plasma etched surface of the hot pressed silicon nitride.
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Fig. 2. Weight loss (DW/S) vs. exposure time at 25°C, 40°C and 70°C during corrosion tests in 1.8 M sulphuric solution. Each experimental point represents the mean value of the tests performed at the speci®c conditions.
Fig. 3. SEM micrograph (by SE) of a surface corroded at 70°C for 200 h.
On the contrary, the damages of the corrosion attack clearly come out from a closer inspection of the cross sections. Examples of them after corrosion tests at 70°C for 20, 200 and 400 h are presented, respectively, in Fig. 4a±c. Similar features were observed in all specimens exposed at various conditions. It can be seen distinctly that grain boundary phases are dissolved all around an external shell of the specimen: its mean thickness, measured from SEM micrographs, increases with temperature and exposure time, with the same tendency exhibited by the weight loss curves.
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Fig. 4. SEM micrographs by backscattered electrons (BSE) from a polished cross-section of sample corroded at 70°C for (a) 20 h, (b) 200 h, (c) 400 h.
The corrosive attack involves the consumption of the grain boundary phases from the external surface of the sample towards the inner bulk: after 20 h at 25°C the outer layer is very thin and not continuous all over the surface (Fig. 5a). The interface
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Fig. 5. SEM micrographs (by BSE) from a polished cross-section of sample corroded at (a) 25°C for 20 h, (b) 40°C for 100 h, (c) 70°C for 400 h.
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Fig. 6. SEM micrograph (by BSE) from a polished and plasma etched cross-section of sample corroded at 70°C for 400 h. Uncorroded material (inside the bulk) encloses such extended region thoroughly depleted of the grain boundary phases.
between the outer (chemically dissolved) shell and the uncorroded bulk results irregular, particularly after longer exposures (>100 h at 40°C and 70°C, Fig. 5b and c). The advancing front of the chemical dissolution of the grain boundary phases proceeds into the bulk intruding through preferential paths (i.e. grain boundary channels) which are no longer leached. After the tests at 70°C several regions with original grain boundary phases completely consumed and widespread porosity were found throughout the bulk (Fig. 6). The relevant leachability of the grain boundary phases from acid aqueous environments likely arises from their own basic nature. In fact Y and La cations behave as network modi®ers of the silicon-oxide±nitride grain boundary glassy phases [23] and confer them a basic character: such compounds are known to be easily attacked by such environments [4,8,24]. In fact the addition of a strong oxidant (K2 Cr2 O7 ) to the test solution did not modify the corrosion behaviour of the material, thus excluding a signi®cative role of redox reactions in case involved. Moreover comparing three inorganic acids (HCl, HNO3 and H2 SO4 ) at 40°C for 200 h similar trends of the corrosion behaviour were obtained (Fig. 7): these results point out that complexing eects and the formation of precipitates have negligible in¯uence on the overall corrosion process. On the other hand, the incomplete dissociation of the sulphuric acid gives a H concentration in solution lower than that in the nitric and hydrochloric aqueous acid solutions, hence enhancing the leachability of the material exposed in the sulphuric aqueous acid environment [5,15,16,20]. In order to de®ne a possible contribution to the corrosion from the dissolution of silicon nitride, besides the dissolution of the grain boundary phases, the weight of La, Y, Si released in the 1.8 M sulphuric solution and measured by ICP±AES was compared to the ®nal weight loss (an example is shown in Fig. 8). The average amount of light elements (N and O) solved from the grain boundary phases was estimated to be about 24 wt.% assuming La5 Si3 NO12 and Y5 Si3 NO12 as composition
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Fig. 7. Weight loss (DW/S) vs. exposure time from cations (Y,La,Si) released during corrosion and measured by ICP±AES in three dierent 3.6 N acid solutions at 40°C for 200 h.
Fig. 8. Comparison of weight loss (DW/S) vs. exposure time measured by the analytical balance (see Fig. 2) and by ICP±AES (cations released in the 1.8 M sulphuric solution) at 40°C.
of the grain boundary phases. The dierence between the ®nal weight loss and the total amount of released La, Y and Si ranges from 21% to 26% under tests at 40± 70°C and from 20% to 22% under tests at 25°C. Therefore, considering that the contribution of N from a complete dissolution of silicon nitride is 40 wt.%, it should
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be excluded that the dissolution of silicon nitride grains signi®cantly contributes to the weight changes during the corrosion tests. Such a hypothesis may be further con®rmed from the comparison of the weight ratio Si= Y La: it ranges from 0.18 to 0.22 under tests at 25°C and from 0.20 to 0.24 under tests at 40°C and 70°C. The ratio calculated under the hypothesis of the aforementioned grain boundary phases composition is 0.18. Thus a contribution of silicon coming from the dissolution of silicon nitride results only just detectable at 70°C, but in any case the available evidences indicate that silicon nitride grains undergo a negligible attack under the adopted corrosion conditions. Therefore the corrosion of silicon-nitride-based material in such a sulphuric acid aqueous solution can be attributed to the leaching of Y and La ions from the grain boundary phases followed by hydrolysis reaction of the residual glassy network. In order to de®ne the rate governing mechanism for the observed corrosion behaviour and its dependence on time and temperature, from the plots in Fig. 9 the following features are drawn: (i) at 40°C and 70°C (from 20 to 400 h of exposure) the slope of the traces is <1, i.e. the dissolution mechanism is hampered by a diusion barrier and the corrosion rate is practically time-independent; (ii) at 25°C up to about 100 h the slope is 1, i.e. the mechanism is a dissolution from the surface, on the contrary after 100 h the corrosion results somehow limited. It can be ascribed to the formation of products, probably La and Y sulphate double salt [9], which, being not completely solubilized, act as a diusion barrier. However, in the present study, it was not possible to reveal any reaction compounds. That being so, the investigation on the corrosion kinetics was deepened as speci®ed in the following. From the amounts of La, Y and Si released in the corrosive solution, a degree of dissolution vi for each component, was calculated:
Fig. 9. Log±log plot of weight loss DW/S vs. exposure time at 25°C, 40°C and 70°C.
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vi A=B
1
where A and B are respectively the weight of the element released in the corrosive solution (measured by ICP±AES) and present in the untreated material. The values of the degree of dissolution for Y and La and the relative trends, as shown in Fig. 10a±c, are very similar and are two order of magnitude higher than that of Si. Considering that silicon is present in the grain boundary phases as well as in the silicon nitride grains, these data con®rm that silicon nitride grains themselves are negligibly attacked. The time±temperature dependence of vi cannot be described with simple relationships. The degrees of dissolution at 25°C increase more than those at 40°C and 70°C for exposure up to 200 h, then for longer times decrease to lower values. In order to verify an agreement of the vi data with speci®c kinetic mechanisms, several kinetic expressions, which describe interfacial chemical reactions or diusion phenomena (transportation of participants to or from a zone of favourite reactions), were evaluated. Regarding tests at 40°C and 70°C from 20 to 400 h, a threedimensional diusion controlled process, i.e. the Ginstlin±Brounsthein model where kGB and t are respectively the reaction rate of the process and the time [25],
Fig. 10. Degree of dissolution vi vs. exposure time of (a) Y, (b) La and (c) Si released in the 1.8 M sulphuric solution at 25°C, 40°C and 70°C. Note that vY and vLa are about two orders of magnitude higher than vSi .
F. Monteverde et al. / Corrosion Science 43 (2001) 1851±1863
1
2 v 3 i
1
vi
2=3
kGB t
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2
accomplished the best ®t of the experimental data among the considered kinetics. At the beginning of tests, a transient regime (about 20 h) occurs during which chemical reactions (probably restricted to fresh exposed surfaces of the sample) are supposed to be the rate controlling step. As the reaction proceeds, mass transport (i.e. the inward diusion of reactants to the reactive sites and/or the removal of products from the interacting interfaces through the grain boundaries) becomes rate limiting, as already previously suggested [24]. The reduction of the volume of the reacting phase progressively leaves only silicon nitride grains and pores (Figs. 5b, c and 6). At 25°C the linear kinetics vi k L t
3
observed up to 100 h indicated that the rate controlling step is the chemical reaction, i.e. a linear dissolution of the reacting species at the reaction interface. The evidence of a linear process emerges from the vi values at 100 h and from the cumulative weight loss which are the highest observed, also in comparison to the dissolution at higher testing temperatures. The rapid build-up of vi may be attributed to the surface exchange between H and exchangeable ions on or near the fresh glassy grain boundaries: no barrier diminishes the eective contact area between reactants and the desorption of La and Y cations at the interface is rate limiting. As above mentioned, the deceleratory trend after 100 h could be ascribed to the formation of undissolvable sulphates. On the basis of the microstructural alterations of samples after corrosion and of the mechanisms governing the process, in¯uences on the phenomenologies due to the formation of pits, pores or defects in the reactant phase (e.g. open channels inside the unreacted material) cannot be excluded. The irregularities of the advancing front and the microstructural inhomogeneities deriving from the selective dissolution of the grain boundary phases may contribute to the kinetics nature of the reaction. We cannot exclude to be in presence of mixed controlling processes, associated to change of the involved mechanisms.
4. Conclusions The corrosion resistance of dense silicon nitride hot pressed with 3 wt.% Y2 O3 3 wt.% La2 O3 as sintering aids in 1.8 M sulphuric solution is primarily related to the selective chemical dissolution of the residual La±Y±Si±O±N glassy grain boundary phases. The microstructure of the bulk interested by the corrosive attack shows only silicon nitride grains with no residual grain boundary phases, being thoroughly consumed by the acid aqueous environment. No reaction products were detected on the surface or in the corroded inner layer of the samples exposed to the aggressive solution.
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The corrosion follows a progressive chemical dissolution of the amorphous grain boundary phases (ion leaching and hydrolysis), with a reaction front which enters irregularly into the bulk and proceeds through preferential paths related to local inhomogeneous distributions of the original grain boundary phases. The corrosion is in¯uenced by the temperature. The weight loss behaviour cannot be described by simple relationships both in function of the temperature and exposure time. Therefore the governing mechanisms were studied by means of measurements of chemical species released in the corrosive solution. Concerning the tests at 40°C and 70°C, a three-dimensional diusion expression provided the best ®tting model for the experimental data: the rate controlling step is the mass transport of the reactants and products through the grain boundary paths. During the tests at 25°C, the kinetics is practically linear up to 100 h (i.e. chemical reactions are ratelimiting). The deceleratory regime observed for longer exposures may be ascribed to the formation of a diusion barrier that should control surface reactions restraining ionic exchange rates between solid and solution ligands and/or the breakdown of the grain boundary network at surface after leaching. Comparing the results from the literature under similar testing conditions, the selected silicon-nitride-based material showed a rather good corrosion resistance in the sulphuric acid aqueous solution. References [1] L. Lewis, Corrosion Resistance of Technical Ceramics, Her Majesty's Stationary Oce, London, UK, 1984. [2] Y.G. Gogotsi, V.A. Lavrenko (Eds.), Corrosion to High Performance Ceramics, Springer, Berlin, 1992. [3] K.G. Nickel (Ed.), Corrosion of Advanced Ceramics, Measurement and Modelling, NATO ASI Series, E-vol 267, Kluwer, Dordrecht, 1994. [4] D.E. Clark, B.K. Zoitos (Eds.), Corrosion of Glass, Ceramics and Ceramic Superconductors, Noyes Publications, Park Ridge, NJ, 1992. [5] T. Sato, Y. Tokunaga, T. Endo, M. Shimada, K. Komeya, M. Komatsu, T. Kameda, J. Am. Ceram. Soc. 71 (12) (1988) 1074. [6] T. Sato, Y. Tokunaga, T. Endo, M. Shimada, K. Komeya, K. Nishida, M. Komatsu, T. Kameda, J. Mater. Sci. 23 (1988) 3440. [7] T. Sato, M. Shimada, Corros. Engng. 37 (1988) 327. [8] R.E. Tressler (Ed.), Corrosion and corrosive degradation of ceramics, Section V. Aqueous Corrosion of Ceramics, Ceramics Transactions, vol. 10, American Ceramic Society, Westerville, OH, 1990. [9] K.A. Nielsen, H.G. Krarup, J.E. Engell, G. Pritzl, Aqueous corrosion of ceramics, in: P. Duran, J.F. Fernandez (Eds.), Third Euro-Ceramics, vol. 3, Faenza Editrice Iberica S.L, Spain, 1993, p. 985. [10] W. Genthe, G. Komp, H. Hausner, Liquid corrosion of SiC ceramics, in: G. Ziegler, H. Hausner (Eds.), Euro-Ceramics II, vol. 2, Deutsche Keramische. Gesellschaft, Koln, Germany, 1992, p. 1315. [11] K.A. Nielsen, J.E. Engell, N. Thomsen, Aqueous corrosion of engineering ceramics, in: G. Ziegler, H. Hausner (Eds.), Euro-Ceramics II, vol. 2, Deutsche Keramische. Gesellschaft, Koln, Germany, 1992, p. 1309. [12] F.A. Costa Oliveira, R.J. Fordham, J.H.W. De Wit, Degradation mechanisms of a silicon nitride in H2 ±H2 O environment, in: G. Ziegler, H. Hausner (Eds.), Euro-Ceramics II, vol. 2, Deutsche Keramische. Gesellschaft, Koln, Germany, 1992, p. 1351. [13] T. Sato, T. Murakami, T. Endo, M. Shimada, K. Komeya, T. Kameda, M. Komatsu, J. Mater. Sci 26 (1991) 1749.
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[14] S. Iio, J. Otsuka, T. Niwa, Y. Tajima, M. Watanabe, Corrosion behaviour of silicon nitride ceramics in sulfuric acid solution, in: M. Mizutani (Ed.), Advanced MaterialsÕ93, I/A: Ceramics, Powders, Corrosion and Advanced Processing, vol. 14A, Trans. Mat. Res. Soc. Jpn., Elsevier, Amsterdam, 1994, p. 215. [15] A. Bellosi, T. Graziani, F. Monteverde, Degradation behaviour of silicon nitride in aqueous acid solutions, in: R.J. Fordham, D.J. Baxter, T. Graziani (Eds.), Corrosion of Advanced Ceramics, Key Engineering Materials, vol. 113, Trans. Tech. Publications, Zurich, Switzerland, 1996, p. 215. [16] K. Komeya, T. Meguro, S. Atago, C.-H.Lin, Y. Abe, M. Komatsu, Corrosion resistance of silicon nitride ceramics, in: K. Niihara, T. Sekino, E. Yasuda, T. Sasa (Eds.), The Science of Engineering Ceramics II, Key Engineering Materials, vols. 161±163, Trans. Tech. Publications, Zurich, Switzerland, 1999, p. 235. [17] A. Okada, M. Yoshimura, Corrosion of advanced ceramics, mechanical degradation of silicon nitride ceramics in corrosive solutions of boiling sulphuric acid, in: R.J. Fordham, D.J. Baxter, T. Graziani (Eds.), Key Engineering Materials, vol. 113, Trans. Tech. Publications, Zurich, Switzerland, 1996, p. 227. [18] T. Yoshio, K. Oda, Corros. Engng. 44 (1995) 481. [19] M. Arai, A. Kohno, Corros. Engng. 44 (1995) 325. [20] S.W. Sharkawy, A.M. El Aslabi, Corros. Sci. 40 (7) (1998) 1119. [21] S. Ono, M. Yuki, M. Kumagai, K. Hoashi, Corros. Engng. 40 (1991) 631. [22] A. Bellosi, F. Monteverde, G.N. Babini, In¯uence of powder treatment methods on sintering, microstructure and properties of Si3 N4 -based materials, in: G.N. Babini, M. Haviar, P. Sajgalik (Eds.), Engineering CeramicsÕ96, NATO ASI Series, vol 25, Kluwer, Dordrecht, 1997, p. 197. [23] M. Ohashi, K. Nakamura, K. Hirao, S. Kanzaki, S. Hampshire, J. Am. Cer. Soc. 78 (1) (1995) 71. [24] R.A. McCauley, in: A. Schwietzer (Ed.), Corrosion of Ceramics, Marcel Dekker, New York, 1995. [25] C.H. Bamford, C.F.H. Tipper (Eds.), Comprehensive Chemical Kinetics, Reactions in the Solid State, vol. 22, Elsevier, Amsterdam, 1980.
www.elsevier.com/locate/corsci
Corrosion of silicon nitride in sulphuric acid aqueous solution F. Monteverde, C. Mingazzini, M. Giorgi, A. Bellosi * CNR-IRTEC, Research Institute for Ceramic Technology, Via Granarolo 64, 48018 Faenza, Italy Received 21 January 2000; accepted 13 May 2000
Abstract The corrosion of a hot pressed silicon nitride (produced with the addition of 3 wt.% Y2 O3 3 wt.% La2 O3 as sintering aids) in 1.8 M sulphuric acid aqueous solution was studied in the temperature range of 25±70°C for times up to 400 h. No reaction products were detected on the exposed surfaces. The corrosion follows a progressive chemical dissolution of the La±Y±Si±O±N grain boundary phases: their basic character, due to the presence of Y and La cations, favours the attack by acid aqueous environments. The reaction front enters irregularly into the bulk and depletes an external layer and inner zones of the samples of grain boundary phases. At 25°C up to 100 h the corrosion kinetics is linear: the chemical reaction is rate limiting. At 40°C and 70°C the kinetics ®ts a three-dimensional diusion law: the mass transport of the reactants to the reaction sites and/or the removal of products from the reacting interface(s) are rate limiting. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Structural ceramic; Silicon nitride; Acid corrosion; Weight change; Corrosion kinetics
1. Introduction In last decades, the applications of silicon-nitride-based materials in many engineering applications, among which corrosion- and wear-resistant components, have gained increasing attention, due to their unique combination of properties: chemical stability, thermal shock resistance, hardness, fracture strength, toughness. A lot of technical and economic resources has been actively applied on high temperature
*
Corresponding author. Tel.: +39-546-699-759; fax: +39-546-46381. E-mail address: [email protected] (A. Bellosi).
0010-938X/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 0 - 9 3 8 X ( 0 0 ) 0 0 1 9 3 - 1
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F. Monteverde et al. / Corrosion Science 43 (2001) 1851±1863
corrosion and oxidation studies of silicon nitride [1±3], whereas fewer papers have concerned the corrosion behaviour at room or moderate temperatures in aqueous corrosive environments [4±21]. Silicon-nitride-based ceramics are multiphase systems with each phase having individual corrosion characteristics and possible reactions at the grain boundaries. It was reported that corrosion of silicon-nitride-based materials varies, depending on the acid or basic conditions and on the residual grain boundary phases of the as-sintered materials (i.e. structure-composition, amount, distribution, degree of crystallinity). As the microstructural features of the grain boundary phases are directly correlated to type, amount of sintering aids and to the adopted manufacturing procedures (powder processing, cold and hot consolidation techniques), various silicon-nitride-based materials showed dissimilar corrosion behaviour and, consequently, dierent eects on structural and functional properties [5,6,9,11,13,14,16±21]. The purpose of the present study is to investigate the corrosion behaviour in 1.8 M sulphuric acid solution of an hot pressed silicon nitride material containing yttria and lanthania as sintering aids. The kinetics features were evaluated and the mechanisms governing the corrosion were de®ned. The in¯uence of the microstructure of the as-sintered material on the corrosion resistance was considered. 2. Experimental procedures A fully dense hot pressed silicon-nitride-based material (containing 3 wt.% Y2 O3 3 wt.% La2 O3 as sintering aids) was employed for corrosion tests. Details on processing and main properties of this material are reported elsewhere [22]. For corrosion tests rectangular plates 10:0 10:0 2:0 mm3 were cut and the wider surfaces were polished up to 6 lm. After polishing, the plates were carefully cleaned in ultrasonicated acetone bath, dried, weighed with an analytical balance (accuracy 0.01 mg) and sealed in a 100 ml polyethylene tube containing 50 ml of 1.8 M sulphuric acid (pH < 0). The tubes were then immersed in a thermostatic bath and gently shaken twice a day. The static corrosion tests were performed at 25°C, 40°C and 70°C for holding times up to 400 h. After the planned exposure time, the specimens were removed from the tubes, rinsed in boiling deionized water, dried in warm heated oven and ®nally weighed. For each experimental condition, at least two tests were performed. The determination of the amount of Si, Y and La ions released in the corrosive solutions was carried out with a Varian-Liberty 200 sequential inductively coupled plasma±atomic emission spectroscopy (ICP±AES). Not withstanding the high acid concentration, adopting a V-groove nebulizer and speci®c conditions (low nebulization pressure and high power applied to the plasma) short-term and long-term stability of signals were both satisfactory. In order to ensure accuracy, matrixmatched standard solutions were prepared. Without any dilution, the concentrations resulted much higher than the (lower) detection limit on the selected analytical lines (La: 408.67 nm; Y: 371.03 nm; Si: 251.61 nm). However the use of a demountable torch with an alumina injector was necessary in order to minimise the signal of
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silicon arising from the quartz parts of the torch itself. The standard solutions were tested over a period of two months to exclude adsorption/precipitation phenomena upon the walls of the tubes. Microstructural investigations of the samples after corrosion were performed by X-ray diraction (XRD) and scanning electron microscopy (SEM) on the surfaces and relative cross sections in order to evaluate the eects of the attack on surface and inside the bulk.
3. Results and discussion The typical microstructure of the hot pressed material is shown in Fig. 1 with elongated and interlocked b-silicon nitride grains (grey areas) and grain boundary phases (bright network). The grain boundary phases are mainly glassy; after annealing treatments La±Y±Si±O±N phases of various stoichiometries were detected by XRD patterns [22]. These results allowed to get informations on the qualitative composition of the residual grain boundary phases of the tested specimens. While neither noticeable dimensional change nor delamination of silicon nitride grains were observed after exposure, all corroded samples suered extensive whitening from the original dark-grey colour all over the surfaces wet by the acid sulphuric aqueous solution. The weight loss data of the corroded samples at 25°C, 40°C and 70°C for holding times up to 400 h are shown in Fig. 2: each tested sample had signi®cant weight loss. XRD patterns from the corroded surfaces did not reveal any additional crystalline phases in comparison to the as-sintered material. On the exposed surfaces only small and mildly leached areas, where grain boundary phases were selectively dissolved, appeared (Fig. 3). The surface aspect was similar in all the samples treated under dierent experimental conditions.
Fig. 1. SEM micrograph by secondary electrons (SE) from a polished and plasma etched surface of the hot pressed silicon nitride.
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Fig. 2. Weight loss (DW/S) vs. exposure time at 25°C, 40°C and 70°C during corrosion tests in 1.8 M sulphuric solution. Each experimental point represents the mean value of the tests performed at the speci®c conditions.
Fig. 3. SEM micrograph (by SE) of a surface corroded at 70°C for 200 h.
On the contrary, the damages of the corrosion attack clearly come out from a closer inspection of the cross sections. Examples of them after corrosion tests at 70°C for 20, 200 and 400 h are presented, respectively, in Fig. 4a±c. Similar features were observed in all specimens exposed at various conditions. It can be seen distinctly that grain boundary phases are dissolved all around an external shell of the specimen: its mean thickness, measured from SEM micrographs, increases with temperature and exposure time, with the same tendency exhibited by the weight loss curves.
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Fig. 4. SEM micrographs by backscattered electrons (BSE) from a polished cross-section of sample corroded at 70°C for (a) 20 h, (b) 200 h, (c) 400 h.
The corrosive attack involves the consumption of the grain boundary phases from the external surface of the sample towards the inner bulk: after 20 h at 25°C the outer layer is very thin and not continuous all over the surface (Fig. 5a). The interface
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Fig. 5. SEM micrographs (by BSE) from a polished cross-section of sample corroded at (a) 25°C for 20 h, (b) 40°C for 100 h, (c) 70°C for 400 h.
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Fig. 6. SEM micrograph (by BSE) from a polished and plasma etched cross-section of sample corroded at 70°C for 400 h. Uncorroded material (inside the bulk) encloses such extended region thoroughly depleted of the grain boundary phases.
between the outer (chemically dissolved) shell and the uncorroded bulk results irregular, particularly after longer exposures (>100 h at 40°C and 70°C, Fig. 5b and c). The advancing front of the chemical dissolution of the grain boundary phases proceeds into the bulk intruding through preferential paths (i.e. grain boundary channels) which are no longer leached. After the tests at 70°C several regions with original grain boundary phases completely consumed and widespread porosity were found throughout the bulk (Fig. 6). The relevant leachability of the grain boundary phases from acid aqueous environments likely arises from their own basic nature. In fact Y and La cations behave as network modi®ers of the silicon-oxide±nitride grain boundary glassy phases [23] and confer them a basic character: such compounds are known to be easily attacked by such environments [4,8,24]. In fact the addition of a strong oxidant (K2 Cr2 O7 ) to the test solution did not modify the corrosion behaviour of the material, thus excluding a signi®cative role of redox reactions in case involved. Moreover comparing three inorganic acids (HCl, HNO3 and H2 SO4 ) at 40°C for 200 h similar trends of the corrosion behaviour were obtained (Fig. 7): these results point out that complexing eects and the formation of precipitates have negligible in¯uence on the overall corrosion process. On the other hand, the incomplete dissociation of the sulphuric acid gives a H concentration in solution lower than that in the nitric and hydrochloric aqueous acid solutions, hence enhancing the leachability of the material exposed in the sulphuric aqueous acid environment [5,15,16,20]. In order to de®ne a possible contribution to the corrosion from the dissolution of silicon nitride, besides the dissolution of the grain boundary phases, the weight of La, Y, Si released in the 1.8 M sulphuric solution and measured by ICP±AES was compared to the ®nal weight loss (an example is shown in Fig. 8). The average amount of light elements (N and O) solved from the grain boundary phases was estimated to be about 24 wt.% assuming La5 Si3 NO12 and Y5 Si3 NO12 as composition
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Fig. 7. Weight loss (DW/S) vs. exposure time from cations (Y,La,Si) released during corrosion and measured by ICP±AES in three dierent 3.6 N acid solutions at 40°C for 200 h.
Fig. 8. Comparison of weight loss (DW/S) vs. exposure time measured by the analytical balance (see Fig. 2) and by ICP±AES (cations released in the 1.8 M sulphuric solution) at 40°C.
of the grain boundary phases. The dierence between the ®nal weight loss and the total amount of released La, Y and Si ranges from 21% to 26% under tests at 40± 70°C and from 20% to 22% under tests at 25°C. Therefore, considering that the contribution of N from a complete dissolution of silicon nitride is 40 wt.%, it should
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be excluded that the dissolution of silicon nitride grains signi®cantly contributes to the weight changes during the corrosion tests. Such a hypothesis may be further con®rmed from the comparison of the weight ratio Si= Y La: it ranges from 0.18 to 0.22 under tests at 25°C and from 0.20 to 0.24 under tests at 40°C and 70°C. The ratio calculated under the hypothesis of the aforementioned grain boundary phases composition is 0.18. Thus a contribution of silicon coming from the dissolution of silicon nitride results only just detectable at 70°C, but in any case the available evidences indicate that silicon nitride grains undergo a negligible attack under the adopted corrosion conditions. Therefore the corrosion of silicon-nitride-based material in such a sulphuric acid aqueous solution can be attributed to the leaching of Y and La ions from the grain boundary phases followed by hydrolysis reaction of the residual glassy network. In order to de®ne the rate governing mechanism for the observed corrosion behaviour and its dependence on time and temperature, from the plots in Fig. 9 the following features are drawn: (i) at 40°C and 70°C (from 20 to 400 h of exposure) the slope of the traces is <1, i.e. the dissolution mechanism is hampered by a diusion barrier and the corrosion rate is practically time-independent; (ii) at 25°C up to about 100 h the slope is 1, i.e. the mechanism is a dissolution from the surface, on the contrary after 100 h the corrosion results somehow limited. It can be ascribed to the formation of products, probably La and Y sulphate double salt [9], which, being not completely solubilized, act as a diusion barrier. However, in the present study, it was not possible to reveal any reaction compounds. That being so, the investigation on the corrosion kinetics was deepened as speci®ed in the following. From the amounts of La, Y and Si released in the corrosive solution, a degree of dissolution vi for each component, was calculated:
Fig. 9. Log±log plot of weight loss DW/S vs. exposure time at 25°C, 40°C and 70°C.
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vi A=B
1
where A and B are respectively the weight of the element released in the corrosive solution (measured by ICP±AES) and present in the untreated material. The values of the degree of dissolution for Y and La and the relative trends, as shown in Fig. 10a±c, are very similar and are two order of magnitude higher than that of Si. Considering that silicon is present in the grain boundary phases as well as in the silicon nitride grains, these data con®rm that silicon nitride grains themselves are negligibly attacked. The time±temperature dependence of vi cannot be described with simple relationships. The degrees of dissolution at 25°C increase more than those at 40°C and 70°C for exposure up to 200 h, then for longer times decrease to lower values. In order to verify an agreement of the vi data with speci®c kinetic mechanisms, several kinetic expressions, which describe interfacial chemical reactions or diusion phenomena (transportation of participants to or from a zone of favourite reactions), were evaluated. Regarding tests at 40°C and 70°C from 20 to 400 h, a threedimensional diusion controlled process, i.e. the Ginstlin±Brounsthein model where kGB and t are respectively the reaction rate of the process and the time [25],
Fig. 10. Degree of dissolution vi vs. exposure time of (a) Y, (b) La and (c) Si released in the 1.8 M sulphuric solution at 25°C, 40°C and 70°C. Note that vY and vLa are about two orders of magnitude higher than vSi .
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1
2 v 3 i
1
vi
2=3
kGB t
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2
accomplished the best ®t of the experimental data among the considered kinetics. At the beginning of tests, a transient regime (about 20 h) occurs during which chemical reactions (probably restricted to fresh exposed surfaces of the sample) are supposed to be the rate controlling step. As the reaction proceeds, mass transport (i.e. the inward diusion of reactants to the reactive sites and/or the removal of products from the interacting interfaces through the grain boundaries) becomes rate limiting, as already previously suggested [24]. The reduction of the volume of the reacting phase progressively leaves only silicon nitride grains and pores (Figs. 5b, c and 6). At 25°C the linear kinetics vi k L t
3
observed up to 100 h indicated that the rate controlling step is the chemical reaction, i.e. a linear dissolution of the reacting species at the reaction interface. The evidence of a linear process emerges from the vi values at 100 h and from the cumulative weight loss which are the highest observed, also in comparison to the dissolution at higher testing temperatures. The rapid build-up of vi may be attributed to the surface exchange between H and exchangeable ions on or near the fresh glassy grain boundaries: no barrier diminishes the eective contact area between reactants and the desorption of La and Y cations at the interface is rate limiting. As above mentioned, the deceleratory trend after 100 h could be ascribed to the formation of undissolvable sulphates. On the basis of the microstructural alterations of samples after corrosion and of the mechanisms governing the process, in¯uences on the phenomenologies due to the formation of pits, pores or defects in the reactant phase (e.g. open channels inside the unreacted material) cannot be excluded. The irregularities of the advancing front and the microstructural inhomogeneities deriving from the selective dissolution of the grain boundary phases may contribute to the kinetics nature of the reaction. We cannot exclude to be in presence of mixed controlling processes, associated to change of the involved mechanisms.
4. Conclusions The corrosion resistance of dense silicon nitride hot pressed with 3 wt.% Y2 O3 3 wt.% La2 O3 as sintering aids in 1.8 M sulphuric solution is primarily related to the selective chemical dissolution of the residual La±Y±Si±O±N glassy grain boundary phases. The microstructure of the bulk interested by the corrosive attack shows only silicon nitride grains with no residual grain boundary phases, being thoroughly consumed by the acid aqueous environment. No reaction products were detected on the surface or in the corroded inner layer of the samples exposed to the aggressive solution.
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The corrosion follows a progressive chemical dissolution of the amorphous grain boundary phases (ion leaching and hydrolysis), with a reaction front which enters irregularly into the bulk and proceeds through preferential paths related to local inhomogeneous distributions of the original grain boundary phases. The corrosion is in¯uenced by the temperature. The weight loss behaviour cannot be described by simple relationships both in function of the temperature and exposure time. Therefore the governing mechanisms were studied by means of measurements of chemical species released in the corrosive solution. Concerning the tests at 40°C and 70°C, a three-dimensional diusion expression provided the best ®tting model for the experimental data: the rate controlling step is the mass transport of the reactants and products through the grain boundary paths. During the tests at 25°C, the kinetics is practically linear up to 100 h (i.e. chemical reactions are ratelimiting). The deceleratory regime observed for longer exposures may be ascribed to the formation of a diusion barrier that should control surface reactions restraining ionic exchange rates between solid and solution ligands and/or the breakdown of the grain boundary network at surface after leaching. Comparing the results from the literature under similar testing conditions, the selected silicon-nitride-based material showed a rather good corrosion resistance in the sulphuric acid aqueous solution. References [1] L. Lewis, Corrosion Resistance of Technical Ceramics, Her Majesty's Stationary Oce, London, UK, 1984. [2] Y.G. Gogotsi, V.A. Lavrenko (Eds.), Corrosion to High Performance Ceramics, Springer, Berlin, 1992. [3] K.G. Nickel (Ed.), Corrosion of Advanced Ceramics, Measurement and Modelling, NATO ASI Series, E-vol 267, Kluwer, Dordrecht, 1994. [4] D.E. Clark, B.K. Zoitos (Eds.), Corrosion of Glass, Ceramics and Ceramic Superconductors, Noyes Publications, Park Ridge, NJ, 1992. [5] T. Sato, Y. Tokunaga, T. Endo, M. Shimada, K. Komeya, M. Komatsu, T. Kameda, J. Am. Ceram. Soc. 71 (12) (1988) 1074. [6] T. Sato, Y. Tokunaga, T. Endo, M. Shimada, K. Komeya, K. Nishida, M. Komatsu, T. Kameda, J. Mater. Sci. 23 (1988) 3440. [7] T. Sato, M. Shimada, Corros. Engng. 37 (1988) 327. [8] R.E. Tressler (Ed.), Corrosion and corrosive degradation of ceramics, Section V. Aqueous Corrosion of Ceramics, Ceramics Transactions, vol. 10, American Ceramic Society, Westerville, OH, 1990. [9] K.A. Nielsen, H.G. Krarup, J.E. Engell, G. Pritzl, Aqueous corrosion of ceramics, in: P. Duran, J.F. Fernandez (Eds.), Third Euro-Ceramics, vol. 3, Faenza Editrice Iberica S.L, Spain, 1993, p. 985. [10] W. Genthe, G. Komp, H. Hausner, Liquid corrosion of SiC ceramics, in: G. Ziegler, H. Hausner (Eds.), Euro-Ceramics II, vol. 2, Deutsche Keramische. Gesellschaft, Koln, Germany, 1992, p. 1315. [11] K.A. Nielsen, J.E. Engell, N. Thomsen, Aqueous corrosion of engineering ceramics, in: G. Ziegler, H. Hausner (Eds.), Euro-Ceramics II, vol. 2, Deutsche Keramische. Gesellschaft, Koln, Germany, 1992, p. 1309. [12] F.A. Costa Oliveira, R.J. Fordham, J.H.W. De Wit, Degradation mechanisms of a silicon nitride in H2 ±H2 O environment, in: G. Ziegler, H. Hausner (Eds.), Euro-Ceramics II, vol. 2, Deutsche Keramische. Gesellschaft, Koln, Germany, 1992, p. 1351. [13] T. Sato, T. Murakami, T. Endo, M. Shimada, K. Komeya, T. Kameda, M. Komatsu, J. Mater. Sci 26 (1991) 1749.
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[14] S. Iio, J. Otsuka, T. Niwa, Y. Tajima, M. Watanabe, Corrosion behaviour of silicon nitride ceramics in sulfuric acid solution, in: M. Mizutani (Ed.), Advanced MaterialsÕ93, I/A: Ceramics, Powders, Corrosion and Advanced Processing, vol. 14A, Trans. Mat. Res. Soc. Jpn., Elsevier, Amsterdam, 1994, p. 215. [15] A. Bellosi, T. Graziani, F. Monteverde, Degradation behaviour of silicon nitride in aqueous acid solutions, in: R.J. Fordham, D.J. Baxter, T. Graziani (Eds.), Corrosion of Advanced Ceramics, Key Engineering Materials, vol. 113, Trans. Tech. Publications, Zurich, Switzerland, 1996, p. 215. [16] K. Komeya, T. Meguro, S. Atago, C.-H.Lin, Y. Abe, M. Komatsu, Corrosion resistance of silicon nitride ceramics, in: K. Niihara, T. Sekino, E. Yasuda, T. Sasa (Eds.), The Science of Engineering Ceramics II, Key Engineering Materials, vols. 161±163, Trans. Tech. Publications, Zurich, Switzerland, 1999, p. 235. [17] A. Okada, M. Yoshimura, Corrosion of advanced ceramics, mechanical degradation of silicon nitride ceramics in corrosive solutions of boiling sulphuric acid, in: R.J. Fordham, D.J. Baxter, T. Graziani (Eds.), Key Engineering Materials, vol. 113, Trans. Tech. Publications, Zurich, Switzerland, 1996, p. 227. [18] T. Yoshio, K. Oda, Corros. Engng. 44 (1995) 481. [19] M. Arai, A. Kohno, Corros. Engng. 44 (1995) 325. [20] S.W. Sharkawy, A.M. El Aslabi, Corros. Sci. 40 (7) (1998) 1119. [21] S. Ono, M. Yuki, M. Kumagai, K. Hoashi, Corros. Engng. 40 (1991) 631. [22] A. Bellosi, F. Monteverde, G.N. Babini, In¯uence of powder treatment methods on sintering, microstructure and properties of Si3 N4 -based materials, in: G.N. Babini, M. Haviar, P. Sajgalik (Eds.), Engineering CeramicsÕ96, NATO ASI Series, vol 25, Kluwer, Dordrecht, 1997, p. 197. [23] M. Ohashi, K. Nakamura, K. Hirao, S. Kanzaki, S. Hampshire, J. Am. Cer. Soc. 78 (1) (1995) 71. [24] R.A. McCauley, in: A. Schwietzer (Ed.), Corrosion of Ceramics, Marcel Dekker, New York, 1995. [25] C.H. Bamford, C.F.H. Tipper (Eds.), Comprehensive Chemical Kinetics, Reactions in the Solid State, vol. 22, Elsevier, Amsterdam, 1980.